1. Field of the Invention
The present invention relates to a picture display apparatus which converts an input light image into a projected picture using a photoelectric effect.
2. Description of the Prior Art
In the conventional picture display apparatus using an electro-optical device, an image displayed on a picture display device is transferred onto a photoconductive layer of an electro-optical device using a typical transfer lens. Although the transferred image quality is not greatly deteriorated using the transfer lens, the transfer lens is big requiring a long transfer distance. This causes the picture display apparatus to be large.
One method to solve the above problem is to directly couple the picture display device and the photoconductive layer using an optical fiber plate composed of bundled optical fibers where the optical fibers have a diameter of several micrometers and different refractive index values which vary from the center to the periphery of the bundle. The transfer lens is eliminated. By using the optical fiber plate, the transfer distance required can be reduced to several centimeters resulting in a smaller picture display apparatus.
Two typical examples of a picture display apparatus using an electro-optical device in accordance with the prior art are explained below. The first transfers an image through a transfer lens 24 and is shown in FIG. 1. The second transfers an image through an optical fiber plate 25. The first apparatus, shown in FIG. 1, has good resolution but requires a long transfer distance between the picture display device 16 and the electro-optical device 8 and a large transfer lens 24 for obtaining good brightness. As a result, the first apparatus is big and expensive. The second apparatus, shown in FIG. 2, can transfer an image in a short distance using an optical fiber plate 25. The aperture of the optical fiber plate 25 can be made large. The resulting picture is bright, however, the picture resolution is remarkably deteriorated unless the image surface of the transparent liquid crystal panel 17 is tightly held in contact with the surface of the optical fiber plate 25. Accordingly, it is necessary to change the glass base 8g of the electro-optical device 8 to an optical fiber plate and form an electro-optical device on the optical fiber plate 25. As a result, because of bad production yield of both optical fiber plates and electro-optical devices, the apparatus is expensive. In addition, when connecting the optical fiber plate of the electro-optical device 8 and the optical fiber plate of the picture display device 16, some moire occurs between adjacent fibers of the bundled optical fiber plate resulting in picture quality deterioration.
An optical fiber plate has three problems. The first problem is deterioration in the transferred image. The aperture rate of the optical fiber plate can be made large but the image surface must be held in tight contact with the optical fiber plate surface to prevent defocussing as a result of a gap between the image surface and the optical fiber plate surface. If a gap is present, the spaces between adjacent fibers of the bundled optical fiber plate appear as a net on the picture which deteriorates the picture quality. In addition, the possibility of generating moire between the net pattern and the displayed picture increases. Moreover, if there is a slant between the fibers and there are a lot of optical fibers bundled together, brightness non-uniformity occurs causing the picture quality to deteriorate remarkably.
The second problem is producing the apparatus and the reliability of the apparatus. As mentioned above, because it is undesirable to place the optical fiber plate apart from the picture plane of the picture display device, the picture plane of the picture display device must be provided directly against the optical fiber plate plane. In addition, the photoconductive layer of the electro-optical device must be attached directly on the optical fiber plate plane. Presently, because the production yield of electro-optical devices is not good, direct coupling between the electro-optical device and the picture display device increases mechanical stress, further lowering the production yield of this part.
The third problem is producing the optical fiber plate. The production man-hour required to produce the optical fiber plate is large because the structure of the optical fiber plate. In addition, the production yield of the optical fiber plate is bad due to breakage or slanting of the optical fibers. As a result, the optical fiber plate has an extremely high cost.